(1) Field of the Invention
This invention relates to the synthesis of novel metal-substituted mesoporous molecular sieves and to their use as a catalysts for peroxide hydroxylation of benzene and oxidation of large substituted aromatics. Specifically the mesoporous molecular sieve catalysts of the present invention are prepared by a neutral S.degree. I.degree. self-assembly method comprising steps of hydrogen H-bonding between neutral amine (S.degree.) or diamine (S.degree.-S.degree.) template and neutral inorganic oxide precursors (I.degree.), followed by hydrolysis and crosslinking under mild reaction conditions. The new templating approach ensures the preparation of hexagonal or hexagonal-like oxidation catalysts exhibiting large framework wall thickness of at least about 17 .ANG., small elementary particle size (.ltoreq.400 .ANG.), and unique combinations of framework-confined uniform mesopores and textural mesopores. In addition, the invention provides for the synthesis of thermally stable pillared lamellar metallosilicates by neutral templating method involving neutral metallosilicate precursors (I.degree.) and neutral diamine surfactants (S.degree.-S.degree.). The invention also provides for efficient recovery and recycling of the neutral template by simple solvent extraction methods. This invention also demonstrates the preparation of transition metal-substituted hexagonal MCM-41 silica using mediated S.sup.+ X.sup.- I.sup.+ templating route and mild reaction conditions.
The invention also relates to a catalytic application of these mesoporous metallosilicates for peroxide oxidation of substituted aromatics with kinetic diameters that are too large (larger than 6 .ANG.) to access the pore structure of the conventional microporous transition metal-substituted silicates such as titano- and vanadosilicalites.
(2) Description of Related Art
One of the most important methods for converting hydrocarbons to useful industrial chemicals, intermediates and pharmaceuticals is catalytic oxidation. Currently, stoichiometric oxidations with inorganic oxidants, such as permanganate and dichromate, are carried out on a large scale in the manufacture of fine chemicals. However, these oxidation routes generate large amounts of waste inorganic salts (pollutants) that are extremely difficult to dispose of and economically impractical to recycle. In addition, these classical oxidation processes exhibit low selectivity and thus involve as an indispensable part of the synthesis a costly separation of the side products. Therefore, there is a growing demand for developing cleaner, catalytic and much more selective alternatives to existing oxidation processes. For example, the classical multistep process of production of hydroquinone from benzene involves the following steps: (i) preparation of aniline from benzene by nitration and reduction in order to generate a functional group that can be easily oxidized; (ii) oxidation with stoichiometric amounts of MnO.sub.2 and (iii) reduction with Fe/HCl (Sheldon R. T. in New Developments in Selective Oxidation, Eds. Centi G. and Trifiro F., Elsevier Sci. Publ. B. V., Amsterdam, (1990) pp. 1-29). The amount of generated waste is huge-about 10 kg of inorganic salts (MnSO.sub.4, FeCl, Na.sub.2 SO.sub.4, NaCl) per kg of hydroquinone product. ##STR1## The current catalytic process on the other hand, uses three steps with the initial step being benzene alkylation to 1,4-diisopropylbenzene followed by catalytic oxidation and acid catalyzed rearrangement of the bis-hydroperoxide. It is estimated that the latter process produces about 10% of inorganic salts formed by the classical process (i.e. &gt;1 kg inorganic salts per kg hydroquinone). It is clear that the classical multistep process leads to huge production of waste by-products. On the other hand the existing catalytic process still generates some waste and organic by-products as a result of oxidatively eliminating the isopropyl groups from the 1,4-diisopropylbenzene. In summary, the disadvantages of the classical method are that it generates large amount of pollutants, the oxidant is difficult if not impossible to recover and the selectivity is very low. On the other hand the disadvantages of the existing catalytic process are that it involves multistep transformations, still generates significant amount of pollutants and difficult to separate by-products. Therefore, a one or two step selective catalytic oxidation process to hydroquinone is highly desirable. Such a catalytic process has been recently disclosed (U.S. Pat. No. 4,410,501) and industrially implemented in Italy by Enichem. The high selectivity toward hydroquinone was achieved by performing a liquid phase peroxide oxidation of phenol in the micropores (approximately 6 .ANG. in size) of the titanium substituted molecular sieve-silicalite (denoted TS-1). Another important advantage of this heterogeneous catalytic system is that the catalyst is stable and can be recovered and recycled.
However, the catalytic oxidation of a much larger organic entities such as 2,6-di-tert-butylphenol (with kinetic diameter of approximately 10 .ANG.) is currently performed only by a homogeneous catalytic routes employing different organometallic complex catalysts such as: Co(disalicyl-idenepropylenetriamine) disclosed by Nishinaga, A. et al. Chem. Lett. 4, 817-820 (1994); binuclear Cu(II).mu.-hydroxo complexes with nitrogen chelating ligands (Mari et al. Chemom. Intell. Lab. Syst. 22(2) 257-263 (1994)); a phase-transfer catalysts such as 18-crown-6, 18-dibenzocrown-6, triethylbenzylammonium chloride (U.S. Pat. No. 1,747,434) and metalloporphyrins or intercalated metalloporphirins such as Cobalt(II) phthalocyaninetetrasulfonate intercalated into a Mg.sub.5 Al.sub.2.5 -layered double hydroxide (LDH)-Chibwe et al., J. Chem. Soc., Chem. Commun. 3, 278-280 (1993). However, the use of homogeneous catalysts has the following major disadvantages: (i) these catalysts are usually very expensive, highly toxic and difficult to separate and to recover from the reaction product and (ii) the catalytic oxidation of the 2,6-di-tertbutylphenol over these metal complexes proceeds primarily to the 3,3',5,5'-tetra(tert-butyl)-4,4'-diphenoquinone dimer rather to the more desirable 2,6-di-tert-butylbenzoquinone monomer, i.e. the selectivity to the corresponding monomer is very low. One way to solve the separation problem, as taught by Chibwe et al., ibid, is to encapsulate these metal complex catalysts in inorganic matrix (such as LDH) and to be able to recover and recycle the catalyst. However, the large scale industrial application of the above processes and the use of expensive and toxic catalysts is still little justified due to the low selectivity toward mono-benzoquinone and separation problems. A very promising way to improve the selectivity toward monomeric benzoquinone would be to limit the size of the active complex, i.e. the size of the reaction product, by performing the oxidation of the large aromatic substrate into the uniform mesopores of a transition metal-substituted porous molecular sieve material.
Porous materials created by nature or by synthetic design have found great utility in all aspects of human activity. The pore structure of the solids is usually formed in the stages of crystallization or subsequent treatment. Depending on their predominant pore size, the solid materials are classified as: (i) microporous, having pore sizes &lt;20 .ANG.; (ii) macroporous, with pore sizes exceeding 500 .ANG.; and (iii) mesoporous, with intermediate pore sizes between 20 and 500 .ANG.. The use of macroporous solids as adsorbents and catalysts is relatively limited due to their low surface area and large non-uniform pores. Microporous and mesoporous solids, however, are widely used in adsorption, separation technology and catalysis. Owing to the need for higher accessible surface area and pore volume for efficient chemical processes, there is a growing demand for new highly stable mesoporous materials. Porous materials can be structurally amorphous, paracrystalline, or crystalline. Amorphous materials, such as silica gel or alumina gel, do not possess long range order, whereas paracrystalline solids, such as .gamma.- or .eta.-Al.sub.2 O.sub.3 are quasiordered as evidenced by the broad peaks on their X-ray diffraction patterns. Both classes of materials exhibit a broad distribution of pores predominantly in the mesoporous range. This wide pore size distribution limits the shape selectivity and the effectiveness of the adsorbents, ion-exchanges and catalysts prepared from amorphous and paracrystalline solids.
The only class of porous materials possessing rigorously uniform pore sizes is that of zeolites and related molecular sieves. Zeolites are microporous highly crystalline aluminosilicates. Their lattice is composed by TO.sub.4 tetrahedra (T=Al and Si) linked by sharing the apical oxygen atoms. Their pore network, which is confined by the spatially oriented TO.sub.4 tetrahedra, consists of cavities and connecting windows of uniform size (Breck D. W., Zeolite Molecular Sieves: Structure, Chemistry and Use; Wiley and Sons; London, 1974). Because of their aluminosilicate composition and ability to discriminate small molecules, zeolites are considered as a subclass of molecular sieves. Molecular sieves are crystalline nonaluminosilicate framework materials in which Si and/or Al tetrahedral atoms of a zeolite lattice are substituted by other T atoms such as B, Ga, Ge, Ti, V, Fe, or P.
Zeolite frameworks are usually negatively charged due to the replacement of Si.sup.4+ by Al.sup.3+. In natural zeolites this charge is compensated by alkali or alkali earth cations such as Na.sup.+, K.sup.+ or Ca.sup.2+. In synthetic zeolites the charge can also be balanced by ammonium cations or protons. Synthetic zeolites and molecular sieves are prepared usually under hydrothermal conditions from alumosilicate or phosphate gels. Their crystallization, according to the hereafter discussed prior art, is accomplished through prolonged reaction in an autoclave for 1-50 days and, often times, in the presence of structure directing agents (templates). The proper selection of template is of extreme importance for the preparation of a particular framework and pore network. A large variety of organic molecules or assemblies of organic molecules with one or more functional groups are known in the prior art to give more than 85 different molecular sieve framework structures. (Meier et al., Atlas of Zeolite Structure Types, Butterworth, London, 1992). An excellent up to date review of the use of various organic templates and their corresponding structures, as well as the mechanism of structure directing is given for example in Gies et al., Zeolites, vol. 12, 42-49 (1992). Due to their uniform pore size, unique crystalline framework structure and ability for isomorphous substitution synthetic zeolites and molecular sieves are extremely suitable for a number of adsorption, separation and catalytic processes involving organic molecules. Recently, it has been discovered that synthetic zeolites and molecular sieves can be functionalized by partially substituting the framework T-atoms with such metal atoms capable of performing different chemical (mostly catalytic) tasks. As a result a large variety of highly selective catalysts have been reported during the last decade. In the spectrum of molecular sieve catalyst a special place is occupied by the metal-substituted, high silica molecular sieves (Si/Al ratio &gt;5). Such molecular sieves are highly hydrophobic and therefore exhibit high affinity toward organic molecules. Among these important materials the microporous Ti-substituted high silica molecular sieve, silicalite-1 (denoted TS-1), with MFI structure and pore size of .apprxeq.6 .ANG. is quickly emerging as a valuable industrial catalyst due to its ability to oxidize organic molecules at mild reaction conditions.
The hydrothermal synthesis of a TS-1 was first disclosed by Taramasso et al in U.S. Pat. No. 4,410,501. According to this prior art TS-1 was prepared by prolonged hydrothermal treatment (175.degree. C. for two to 10 days) of a reaction mixture consisting of a tetraethylorthosilicate (TEOS) as a source of silica, tetraethylorthotitanate (TEOT) as a source of Ti and tetrapropylammonium hydroxide (TPAOH) as a template or structure directing agent. However, a positively charged quaternary ammonium ion template (S.sup.+) was used in order to assemble the Ti-silicalite framework by base catalyzed hydrolysis of the inorganic alkoxides. The base catalyzed hydrolysis of the inorganic alkoxide affords a pentacoordinated transition state (via a nucleophilic attack on the T atom by OH.sup.-), giving T(alkoxy).sub.4 OH!.sup.-, and finally reactive intermediate of the type T(alkoxy).sub.4-x (OH).sub.x. Thus, the silicalite framework is most likely formed by condensation of I.sup.- species of the type TO.sub.x (OR).sub.4-x !.sup.- (where R is alkyl or hydrogen) around single quaternary ammonium cations (S.sup.+). The charge balance in the final templated product is achieved by coupling of the S.sup.+ and OH.sup.- ion pairs in the cavity of silicalite framework with only van der Waals interactions existing between the neutral silicate framework and the occluded template. The preparation of TS-1, as described in Example 2 of this prior art, clearly involves S.sup.+ I.sup.- electrostatic templating forces between the cationic quaternary ammonium template (S.sup.+) and the negatively charged silica precursor (I.sup.-), such as ammonium stabilized LUDOX TM-AS-40 from DuPont Inc. Example 8 represents yet another illustration of the S.sup.+ I.sup.- TS-1 preparation with the negative charge on the inorganic oxide precursor coming from the partial substitution of Si.sup.4+ for Al.sup.3+. Numerous reports in the recent literature (see for example Davis et al, J. Phys. Chem. 98, 4647-4653 (1994)) point out that the charged TPA.sup.+ template species are strongly geometrically (and electrostaticly in the case of Al-substituted TS-1) confined to the cavities of the silicalite framework and that the only way to remove the template is to destroy it by calcination of the crystalline material in air at 550.degree. C. The 96 tetrahedra per unit cell of silicalite propagate in 3 dimensions to reveal a system of intersecting framework-confined micropores composed of 10-membered parallel elliptical (5.1.times.5.7 .ANG.) channels along 100! and zig-zag nearly circular (5.4.+-.0.2 .ANG.) channels along 010!. The specific surface area and total pore volume of TS-1 are usually in the range of 328 to 485 m.sup.2 /g and 0.096 to 0.136 g/g (n-hexane), respectively. The full N.sub.2 adsorption-desorption isotherms of TS-1 exhibits typical Langmuir character. Occasionally, depending on the preparation conditions, a poorly developed hysteresis loop in the Pi/Po &gt;0.4 region is observed. This hysteresis loop most likely corresponds to capillary condensation in nonuniform textural mesopores. However, the presence of textural mesopores would not significantly influence the catalytic oxidation properties of TS-1 toward large substituted aromatics (with kinetic diameters &gt;6 .ANG.) since they can not penetrate into the framework-confined micropores and undergo catalytic transformation. The size of the framework-confined micropores of TS-1 is equal to the size of the parallel elliptic and intersecting zig-zag channels (.sup..about. 6 .ANG.).
TS-1 was found to be an effective oxidation catalyst for a variety of organic compounds using aqueous hydrogen peroxide as oxidant. Prior art examples include oxidation of alkanes (P. A. Jacobs et al, Nature, 345, 240-242 (1990)), oxidation of primary alcohols to aldehydes and secondary alcohols to ketones (U.S. Pat. No. 4,480,135), epoxidation of olefins (Eur. Pat. No. 100,119), hydroxylation of aromatic compounds (G.B.R. Pat. No. 2,116,974 and Tangaraj et al, Appl. Catal. 57 (1990) L1) and oxidation of aniline (Tuel et al., Appl. Catal., A: 118(2) 173-186 (1994)). It is speculated that the catalytic activity of the TS-1 is related directly to the presence of site isolated titanium in the silicate framework. However, because of the small pore size the number of the organic compounds that can be oxidized by TS-1 is strongly limited to molecules having kinetic diameters equal to or less than about 6 .ANG.. Another titanium silicalite, TS-2, with MEL structure was recently reported to exhibit similar oxidation properties (Reddy et al, J. Catal., 130, 440-446 (1991)). The preparation of this molecular sieve again involved the use of charged (S.sup.+) quaternary ammonium template (TBA.sup.+) and prolonged hydrothermal conditions. The similar catalytic behavior of TS-2 is not surprising in view of the nearly identical size of the silicalite-2 framework-confined micropore channels (.sup..about. 5.3 .ANG.). Very recently, a Ti-substituted analog of yet another zeolite (zeolite .beta.) with slightly larger micropore size has been disclosed by Corma et al., J. Chem. Soc. Chem. Commun., 589-590 (1992). The synthesis of this Al.sup.3+ containing zeolite was accomplished by hydrothermal treatment of a reaction mixture containing again an ionic reactants such as Al(NO.sub.3).sub.3, amorphous silica (which at the alkaline pH of the synthesis will slowly dissolve giving most likely a negatively charged (I.sup.-) silica species) and a cationic template (TEAOH). Therefore, this oxidation catalyst can also be classified as prepared by the S.sup.+ I.sup.- electrostatic templating approach. The main incentive for preparing Ti-substituted analog of zeolite b was to be able to take advantage of its slightly larger micropore size pore network composed by intersecting 12-membered ring (7.6.times.6.4 .ANG.) channels along 001! and 12-membered channels (5.5 .ANG.) along 100!. However, the catalytic oxidation chemistry of Ti-substituted zeolite .beta., with the exeption of the slightly higher conversion toward cyclododecane than TS-1, was again confined to the well known small substrates subjectable to catalytic oxidation over TS-1 and TS-2 molecular sieves. In addition, the presence of Al.sup.3 + in the zeolite .beta. affords a hydrophilic framework exhibiting much higher acidity than TS-1 and TS-2. This precludes the possibility for catalytic oxidations of bulky alkyl substituted aromatics or phenols without dealkylation of the alkyl groups. The small micropore size of two recently discovered Ti-substituted molecular sieves, both prepared by electrostatic templating, namely ETS-10 (Anderson et al., Nature, 367, 347-351 (1994)) and Ti-ZSM-48 (Davis et al., J. Chem. Soc. Chem. Commun. 745-747 (1992)), would most likely confine their catalytic oxidation chemistry again to substrates with kinetic diameters smaller than 6 .ANG..
Finally, V-substituted silicalite-1 and 2 (denoted VS-1 and VS-2) oxidation catalysts were also reported very recently (see for example Reddy et al., Catal. Lett. 28, 263-267 (1994) and Rao et al, J. Catal. 141(2) 604-611 (1993)). However, due to the embedding of V in the same silicalite microporous framework the catalytic oxidation activity of these molecular sieves was again limited to small organic substrates with kinetic diameters of less than 6 .ANG..
In summary, all prior art microporous molecular sieve peroxide oxidation catalysts are prepared by S.sup.+ I.sup.- templating route involving electrostatic interactions between positively charged templates (S.sup.+) and negatively charged inorganic oxide precursors (I.sup.-). The condensation of the microporous framework was accomplished by a prolonged hydrothermal treatment around a single charged surfactant species (S.sup.+). Due to the strong electrostatic interactions (for Al.sup.3+ substituted materials) or geometric confinement (for pure Si materials) the charged template is strongly bonded or occluded into the microporous framework and impossible to recover. Finally, due to the small framework micropore size of the prior art molecular sieves the site isolated transition metal centers (such as Ti or V) were accessible by and active only for peroxide oxidations of small organic molecules (such as alkanes, cycloalkanes, alcohols, olefins, benzene, phenol or aniline with kinetic diameters less that about 6 .ANG.).
Therefore, there is a need for a new transition metal-substituted mesoporous molecular sieves capable of catalyzing the oxidation of organic species with kinetic diameters &gt;6 .ANG., especially substituted aromatics. Such transition metal-substituted mesoporous molecular sieves would greatly complement and extend the catalytic chemistry of prior art titanium and vanadium silicalites toward much larger aromatic compounds.
A breakthrough toward the preparation of mesoporous molecular sieves have been disclosed recently in U.S. Pat. Nos. 5,098,684; 5,102,643. The claimed class of mesoporous materials (denoted as M41S) was found to possess uniform and adjustable pore size in the range of 13-100 .ANG.. In addition, these materials exhibited a small framework wall thickness of from 8 to 12 .ANG. and elementary particle size of usually much larger than 500 .ANG.. Depending on preparation conditions M41S materials with hexagonal (MCM-41), cubic (MCM-48) or layered crystallographic structure have been disclosed (Beck et al., J. Am. Chem. Soc., vol. 114, 10834-10843 (1992). The most regular preparations of this prior art give an X-ray diffraction pattern with a few maxima in the extreme low angle region. The positions of these maxima in the case of MCM-41 fit the positions of the hkO reflections of the hexagonal lattice (namely 100, 110, 200 and 210). In addition to that these materials show very characteristic electron diffraction pattern with approximately hexagonal arrangement of diffraction maxima. The postulated mechanism of formation of these molecular sieves involves strong electrostatic interactions and ion pairing between quaternary ammonium liquid crystal cations, as structure directing agents, and anionic silicate oligomer species (U.S. Pat. No. 5,098,684). Related mesoporous structures also have been prepared by rearrangement of a layered silicate (kanemite) (Inagaki et al., J. Chem. Soc. Chem. Commun., vol. 8, 680-682 (1993)) in the presence of quaternary ammonium cations. Recently, Stucky et al. (Nature, vol. 368, 317-321 (1994)) extended the electrostatic assembly approach by proposing four complementary synthesis pathways. Pathway 1 involved the direct co-condensation of anionic inorganic species (I.sup.-) with a cationic surfactant (S.sup.+) to give assembled ion pairs (S.sup.+ I.sup.-), the original synthesis of MCM-41 being the prime example (U.S. Pat. No. 5,098,684). In the charge reversed situation (Pathway 2) an anionic template (S.sup.-) was used to direct the self-assembly of cationic inorganic species (I.sup.+) via S.sup.- I.sup.+ ion pairs. The pathway 2 has been found to give a hexagonal iron and lead oxide and different lamellar lead and aluminum oxide phases (Stucky et al., ibid). Pathways 3 and 4 involved counterion (X.sup.- or M.sup.+) mediated assemblies of surfactants and inorganic species of similar charge. These counterion-mediated pathways afforded assembled solution species of type S.sup.+ X.sup.- I.sup.+ (e.g., X.sup.- =Cl.sup.-, Br.sup.-) or, S.sup.- M.sup.+ I.sup.- (e.g., M.sup.+ =Na.sup.+, K.sup.+), respectively. The viability of Pathway 3 was demonstrated by the synthesis of a hexagonal MCM-41 using a quaternary ammonium cation template and strongly acidic conditions (5-10M HCl or HBr) in order to generate and assemble positively-charged framework precursors (Stucky et al., ibid). In another example, a condensation of anionic aluminate species was accomplished by alkali cation mediated (Na.sup.+, K.sup.+) ion pairing with an anionic template (C.sub.12 H.sub.25 OPO.sub.3.sup.-). The preparation of the corresponding lamellar Al(OH).sub.3 phase in this case has been attributed to the fourth pathway (S.sup.- M.sup.+ I.sup.-). The preparation of Ti-substituted analog of MCM-41 was first demonstrated by Corma et al., J. Chem. Soc. Chem Commun. 147-148 (1994). However, this prior art applies the S.sup.+ I.sup.- (Pathway 1) templating route and prolonged hydrothermal synthesis conditions in order to prepare the Ti-MCM-41 analog. In addition, the catalytic activity of this particular material was illustrated by the epoxidation of rather small organic molecules such as hex-1-ene (in the presence of H.sub.2 O.sub.2) and the tertbutyl peroxide oxidation of norbornene. Simultaneously, we have reported (Pinnavaia et al., Nature, vol. 368, 321-323 (1994)) the preparation of a hexagonal or hexagonal-like mesoporous silica molecular sieve and a Ti-substituted analog (Ti-HMS) by acid catalyzed hydrolysis of inorganic alkoxide precursors in the presence of a partially protonated primary amine surfactants (S.degree./S.sup.+). In the same work we have also demonstrated the first ambient temperature preparation of Ti-MCM-41 molecular sieve using different S.sup.+ X.sup.- I.sup.+ (Pathway 3) templating route. We also reported that both Ti-HMS and Ti-MCM-41 exhibit remarkable catalytic activity toward peroxide oxidation of very large aromatic substrates such as 2,6-DTBP. Here the term "hexagonal" was selected to describe the materials that exhibit in addition to the first d.sub.100 reflection at least some of the remaining reflections of the hexagonal phase or a diffuse scattering centered where these reflections (namely 110, 200, 210, etc.) are expected. In this materials the rod-like uniform channels are assembled in such a fashion that each channel in most cases is surrounded by six neighboring channels.
The term "hexagonal-like" was chosen to describe materials that exhibit single d.sub.100 reflections and no additional reflections or diffuse scattering on their diffraction patterns where the remaining 110, 210 and 200 reflections of the hexagonal phase are expected. This term was also selected to describe materials with deviations from the perfect hexagonal symmetry due to very small scattering domain sizes (.ltoreq.400 .ANG.) or due to crystallographic defects or combinations thereof. In this materials the rod-like uniform channels are assembled in such a fashion that each channel is usually surrounded by less than six neighbors due to lattice defects or to small particle size or combination thereof.
Very recently we also have demonstrated (Tanev and Pinnavaia, Science, 267:865-867 (1995)) a new templating route to mesoporous molecular sieves based on H-bonding and self-assembly between neutral primary amine or diamine surfactants (S.degree.) and neutral inorganic precursors (I.degree.) at ambient temperature. Our new S.degree. I.degree. templating approach, which we denoted a Pathway 5, is complementary to the above electrostatic templating pathways. When we applied the S.degree. I.degree. pathway to the synthesis of hexagonal or hexagonal-like mesoporous silicates (denoted HMS) using a neutral primary amine as the template and tetraethyl orthosilicate as the inorganic reagent we obtained derivatives with thicker framework walls, smaller X-ray scattering domain size, and substantial textural mesoporosity, relative to MCM-41 analogs prepared by an electrostatic S.sup.+ I.sup.- or S.sup.+ X.sup.- I.sup.+ templating pathway. The thicker pore walls are highly desired as improving the thermal and hydrothermal stability (Coustel et al., J. Chem. Soc., Chem. Commun., 967-968 (1994)) of the mesopore framework. The pore walls of the prior art MCM-41 and our HMS materials, as revealed by X-ray diffraction measurements, do not exhibit any crystallographic order, i.e. behave like "amorphous" phase. One skilled in the art will know that thicker "amorphous" walls can give a rise to microporous cavities capable to accommodate small organic molecules. Also, one skilled in the art will know that this could be very important for instance in oxidation of small organic substrates such as benzene. The small particle size and substantial textural mesoporosity are essential for accessing the framework-confined pores and for improving the performance of the obtained adsorbents and catalysts (Pinnavaia et al., ibid; Chavin et al., J. Catal., vol. 111, 94-105 (1988)). In addition, the S.degree. I.degree. pathway allowed for facile recovery of the template by simple solvent extraction, circumventing the need for cation donors or ion pairs to remove the charged template.
Most prior art templating pathways are based on charge matching between ionic organic directing agents and ionic inorganic reagents. The template is strongly bonded to the charged framework and difficult to recover. In the original Mobil approach (U.S. Pat. No. 5,098,684) the template was not recovered, but simply burned off by calcination at elevated temperatures. Recently, it has been demonstrated that the ionic surfactant in Pathway 1 materials could be removed by ion-exchange with acidic cation donor solution (U.S. Pat. No. 5,143,879). Also, the template-halide ion pairs in the framework of acidic Pathway 3 materials were displaced by ethanol extraction (Stucky et al., ibid). Thus, ionic template recovery is possible, provided that exchange ions or ion pairs are present in the extraction process.
Here we disclose a new templating route to transition metal-substituted mesoporous molecular sieves that is complementary to Pathways 1 to 4. Our approach is based on H-bonding and self-assembly between neutral primary amine micelles (S.degree.) and neutral inorganic precursors (I.degree.). This new S.degree. I.degree. templating route, which we denote Pathway 5, affords mesoporous metallosilicates with larger wall thicknesses, small particle sizes and complementary textural mesoporosities relative to Pathway 1 and 3 materials. Both HMS and MCM-41 silicates offer exciting opportunities for the preparation of large pore analogs of the industrially important TS-1 catalyst. We also disclose the catalytic activity of these Ti-HMS and Ti-MCM-41 (prepared by a S.sup.+ X.sup.- I.sup.+ templating route) derivatives for the selective peroxide oxidation of substrates that are too large to access the micropore framework of conventional TS-1, TS-2, VS-1 or VS-2.
Hereafter, in order to clarify one of the objects of the present invention, we would like to define and differentiate the terms framework-confined uniform porosity and textural porosity. Framework-confined uniform pores are pores formed by nucleation and crystallization of the framework elementary particles. These pores typically are cavities and channels confined by the solid framework. The size of the cavities and channels, i.e. the size of the framework-confined uniform pores, in molecular sieve materials is highly regular and predetermined by the thermodynamically favored assembly routes. The framework-confined pores of freshly crystallized product are usually occupied by the template cations and water molecules. While water molecules are easily removed by heating and evacuation the quaternary ammonium cations, due to their high charge density, are strongly bonded or confined to the pore cavities and channels of the negatively charged framework. The same concepts are expected to apply for the charge reversed situation where an anionic template is confined in the pores of a positively-charged framework. Therefore, a cation or anion donor or ion pairs are necessary in order to remove the charged template from the framework of the prior art molecular sieves.
Textural porosity is the porosity that can be attributed to voids and channels between elementary particles or aggregates of such particles (grains). Each of these elementary particles in the case of molecular sieves is composed of certain number of framework unit cells or framework-confined uniform pores. The textural porosity is usually formed in the stages of crystal growth and segregation or subsequent thermal treatment or by acid leaching. The size of the textural pores is determined by the size, shape and the number of interfacial contacts of these particles or aggregates. Thus, the size of the textural pores is usually at least one or two orders of magnitude larger than that of the framework-confined pores. For example, the smaller the particle size, the larger the number of particle contacts, the smaller the textural pore size and vice versa. One skilled in the art of transmission electron spectroscopy (TEM) could determine the existence of framework-confined micropores from High Resolution TEM (HRTEM) images or that of framework-confined mesopores from TEM images obtained by observing microtomed thin sections of the material as taught in U.S. Pat. No. 5,102,643.
One skilled in the art of adsorption could easily distinguish and evaluate framework-confined uniform micropores by their specific adsorption behavior. Such materials usually give a Langmuir type (Type I) adsorption isotherm without a hysteresis loop (Sing et al., Pure Appl. Chem., vol. 57, 603-619 (1985)). The existence of textural mesoporosity can easily be determined by one skilled in the art of SEM, TEM and adsorption. The particle shape and size can readily be established by SEM and TEM and preliminary information concerning textural porosity can also be derived. The most convenient way to detect and assess textural mesoporosity is to analyze the N.sub.2 or Ar adsorption-desorption isotherm of the solid material. Thus, the existence of textural mesoporosity is usually evidenced by the presence of a Type IV adsorption-desorption isotherm exhibiting well defined hysteresis loop in the region of relative pressures Pi/Po &gt;0.4 (Sing et al., ibid). This type of adsorption behavior is quite common for a large variety of paracrystalline materials and pillared layered solids.
The microporous transition metal-substituted zeolites and molecular sieves of the prior art exhibit mainly framework-confined uniform micropores, and little or no textural mesoporosity as evidenced by their Langmuir type adsorption isotherms accompanied with poorly developed hysteresis loops at Pi/Po &gt;0.4. The typical values for their specific surface area are from 300-500 m.sup.2 /g and for the total pore volume .ltoreq.0.6 cm.sup.3 /g (Perspectives in Molecular Sieve Science, Eds. Flank, W. H. and White T. E. Jr., ACS symposium series No. 368, Washington D.C., p. 247; 524; 544 (1988)). All known microporous high silica metallosilicates are prepared by prolonged crystallization at hydrothermal conditions, using single quaternary ammonium cations or protonated primary, secondary or tertiary amines (S.sup.+) to assemble the anionic inorganic species (I.sup.-) into a microporous framework. It should also be noted that the use in the prior art of neutral amines and alcohols as templates (Gunnawardane et al., Zeolites, vol. 8, 127-131 (1988)) has led to the preparation of only microporous highly crystalline (particle size &gt;2 .mu.m) molecular sieves that lack appreciable textural mesoporosity. For the mesoporous molecular sieves of the MCM-41 family the uniform mesopores are also framework-confined. This has been verified by TEM lattice images of MCM-41 shown in U.S. Pat. No. 5,102,643. Therefore, the framework of this class of materials can be viewed as a expanded version of a hexagonal microporous framework. The existence of these framework-confined uniform mesopores was also confirmed by the capillary condensation phenomenon observed in their adsorption isotherms. Typical N.sub.2 adsorption-desorption isotherm of MCM-41, prepared by S.sup.+ I.sup.- templating method (Pathway 1), (Davis et al., XIII North American Meeting of the Catalysis Soc., Book of Abstracts, p. D14 (1993)) is included here for reference (FIG. 1). This adsorption isotherm is essentially the same as that obtained previously by Sing et al., J. Chem. Soc., Chem. Commun., 1257-1258 (1993). The isotherm is characterized by a sharp adsorption uptake followed by a hysteresis loop in the Pi/Po region of 0.3 to 0.4. This hysteresis corresponds to capillary condensation into the framework-confined uniform mesopores. The lack of appreciable hysteresis beyond Pi/Po &gt;0.4 implies the absence of textural mesoporosity. This lack of textural mesoporosity is also supported in some cases by the highly ordered hexagonal prismatic shaped aggregates of size &gt;2 .mu.m (Beck et al., J. Am. Chem. Soc., vol. 114, 10834-10843 (1992). The total pore volume of the material reported by Davis et al. is .apprxeq.0.7 cm.sup.3 /g and that of the framework-confined mesopores, as determined from the upper inflection point of that hysteresis loop, is almost equal to that of the total pore volume. Therefore, the ratio of textural to framework-confined mesoporosity here approaches zero. The size of the framework-confined uniform mesopores is .apprxeq.30 .ANG..
Thus, the metallosilicate molecular sieve materials of the aforementioned prior art typically lack appreciable textural mesoporosity. However, there is increasing number of reports in the literature suggesting that textural mesopores behave as a transport pores to the framework-confined uniform pores and that they greatly improve the access and the performance of adsorbents, ion-exchangers and catalysts. This, for example, is demonstrated by Pinnavaia et al., Nature, vol. 368, 321-323 (1994); Chavin et al., J. Catal., vol. 111, 94-105 (1988) and in Cartlidge et al., Zeolites, vol. 9, 346-349 (1989). According to this prior art the transport pores provide more efficient assess to the framework-confined pores of the molecular sieve.
In summary, the prior art transition metal-substituted molecular sieves, as well as their preparation approaches have the following disadvantages:
1. The prior art uses charged surfactant ions (S.sup.+ or S.sup.-) as templates in order to assemble an inorganic molecular sieve framework from charged inorganic oxide precursors (I.sup.- or I.sup.+). These charged templates are usually expensive, strongly bonded or geometrically confined to the charged inorganic oxide framework and difficult to recover. In all the prior art examples the electrostatically bonded templates were removed from the framework by either a burning off process or by an ion-exchange reaction with an ion donor solution. Also, ion pairs were necessary in order to extract the template from the framework of Pathway 3 materials.
2. The use of charged templates and hydrothermal synthesis conditions afford the preparation of microporous and mesoporous molecular sieves with large elementary particle size (usually much above 500 .ANG.) and absence of optional balance of framework-confined and textural porosity. This does not contribute to accessing the framework-confined uniform pores. The lack of textural mesoporosity could lead to serious diffusional limitations in many potential applications. The ratio of textural to the framework-confined porosity of these materials is usually close to zero.
3. Another important disadvantage of the prior art mesoporous molecular sieves is their small framework wall thickness (from 8 to 12 .ANG.). This does not contribute to improving the thermal and hydrothermal stability of these prior art materials.
4. Due to the small pore size of the prior art microporous molecular sieve frameworks (such as metallosilicalites) the site isolated transition metal centers (such as Ti or V) were accessible and active only for peroxide oxidation of small organic molecules (such as alkanes, cycloalkanes, alcohols, olefins, benzene, phenol or aniline) with kinetic diameters of less that about 6 .ANG..
5. The catalytic oxidation of a large substituted aromatics, such as 2,6-di-tert-butylphenol, (with kinetic diameters of approximately 10 .ANG.) is currently performed by a homogeneous catalytic routes employing different organometallic complex catalysts. However, the use of homogeneous catalysts has the following major disadvantages: (i) these catalysts are usually very expensive, highly toxic and difficult to separate and recycle from the reaction product and (ii) the catalytic oxidation of the 2,6-di-tert-butylphenol over these metal complexes proceeds with low selectivity to the 2,6-di-tert-butylbenzoquinone monomer. Therefore, the large scale industrial application of these expensive and toxic catalysts is little justified.
The aforementioned disadvantages of the prior art severely limit the potential industrial applications of these catalytic materials.
Therefore, there is a need for a new metal-substituted mesoporous molecular sieves capable of selectively catalyzing the oxidation of much larger organic species with kinetic diameters &gt;6 .ANG., especially substituted aromatics or polyaromatics. Such transition metal-substituted mesoporous molecular sieves would greatly complement and extend the catalytic chemistry of prior art microporous titanium and vanadium silicates toward large aromatic molecules. In addition, there is a need for such mesoporous molecular sieve structures exhibiting high thermal and hydrothermal stability (large framework wall thickness), small particle size and complementary framework-confined and textural mesoporosity. Also there is a need for a new preparation art to these ordered mesostructures which would allow for cost reduction by employing less expensive reagents and mild reaction conditions while at the same time providing for the effective recovery and recyclability of the neutral template.